From light to heavy alkali metal tetraphosphonates (M = Li, Na, K, Rb, Cs): cation size-induced structural diversity and water-facilitated proton conductivity

Abstract

A family of alkali metal-based frameworks containing the tetraphosphonate ligand hexamethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid), HDTMP, is reported. A cation size-induced structural diversity, from monodimensional solids (Li⁺ and Na⁺) through layered (K⁺) to pillared-layered (Rb⁺ and Cs⁺) structures, was found. The proton conductivity properties of the Li compounds (hydrated and dehydrated) are reported and the influence of dehydration/rehydration processes in enhancing proton transfer processes is highlighted. Reversible changes in the dimensionality occurred upon full dehydration/rehydration with minor rearrangements in the framework, implying variations in the Li⁺–ligand connectivity but preserving the tetracoordination of the metal ion. The reversibly dehydrated–rehydrated sample displayed the highest proton conductivity (5 × 10⁻³ S cm⁻¹ at 80 °C and 95% RH), a behavior attributed to reversible formation/reformation of P–O(H)–Li bonds that, in turn, provoked changes in the acidity of acid groups and water mobility in the temperature range of impedance measurements.